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Technical Brief

Computational and Experimental Fatigue Analysis of Contoured Spinal Rods

[+] Author and Article Information
Agnese Piovesan, Francesca Berti, Giancarlo Pennati

Laboratory of Biological Structure Mechanics,
Department of Chemistry, Materials and
Chemical Engineering “Giulio Natta”,
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milan 20133, Italy

Tomaso Villa

Laboratory of Biological Structure Mechanics,
Department of Chemistry, Materials and
Chemical Engineering “Giulio Natta”,
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milan 20133, Italy

Luigi La Barbera

Laboratory of Biological Structure Mechanics,
Department of Chemistry, Materials and
Chemical Engineering “Giulio Natta”,
Politecnico di Milano,
Piazza Leonardo da Vinci 32,
Milan 20133, Italy
e-mail: luigi.labarbera@polimi.it

1Agnese Piovesan and Francesca Berti contributed equally.

2Corresponding author.

Manuscript received October 5, 2018; final manuscript received January 29, 2019; published online February 25, 2019. Assoc. Editor: Anton E. Bowden.

J Biomech Eng 141(4), 044505 (Feb 25, 2019) (6 pages) Paper No: BIO-18-1438; doi: 10.1115/1.4042767 History: Received October 05, 2018; Revised January 29, 2019

Posterior fixation with contoured rods is an established methodology for the treatment of spinal deformities. Both uniform industrial preforming and intraoperative contouring introduce tensile and compressive plastic deformations, respectively, at the concave and at the convex sides of the rod. The purpose of this study is to develop a validated numerical framework capable of predicting how the fatigue behavior of contoured spinal rods is affected by residual stresses when loaded in lordotic and kyphotic configurations. Established finite element models (FEM) describing static contouring were implemented as a preliminary simulation step and were followed by subsequent cyclical loading steps. The equivalent Sines stress distribution predicted in each configuration was compared to that in straight rods (SR) and related to the corresponding experimental number of cycles to failure. In the straight configuration, the maximum equivalent stress (441 MPa) exceeds the limit curve, as confirmed by experimental rod breakage after around 1.9 × 105 loading cycles. The stresses further increased in the lordotic configuration, where failure was reached within 2.4 × 104 cycles. The maximum equivalent stress was below the limit curve for the kyphotic configuration (640 MPa), for which a run-out of 106 cycles was reached. Microscopy inspection confirmed agreement between numerical predictions and experimental fatigue crack location. The contouring technique (uniform contouring (UC) or French bender (FB)) was not related to any statistically significant difference. Our study demonstrates the key role of residual stresses in altering the mean stress component, superposing to the tensile cyclic load, potentially explaining the higher failure rate of lordotic rods compared to kyphotic ones.

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Figures

Grahic Jump Location
Fig. 1

Work flow of computational (FEM) and experimental (Exp.) analysis: preliminary static contouring steps (a) based on French bender (FB) and uniform four-point bending contouring (UC) [8]; (b) subsequent fatigue steps for straight and contoured spinal rods according to lordotic and kyphotic configurations

Grahic Jump Location
Fig. 2

(a) Constant life diagram at 1 × 106 cycles for the SR: limit curves are shown for K = 0.5 (solid line) and K = 0.7 (dashed line); (b) equivalent Sines stress map, with maximum value pointed out by an arrow; and (c) experimental failure mechanism

Grahic Jump Location
Fig. 3

(a) Constant life diagrams at 1 × 106 cycles for the uniform contouring specimens, in both the lordotic (UCL) and kyphotic (UCK) configurations: limit curves are shown for K = 0.5 (solid line) and K = 0.7 (dashed line); (b) equivalent Sines stress maps, with maximum value pointed out by an arrow; and (c) experimental failure mechanism, if any

Grahic Jump Location
Fig. 4

(a) Constant life diagrams at 1 × 106 cycles for the French bender specimens, in both the lordotic and kyphotic configurations: limit curves are shown for K = 0.5 (solid line) and K = 0.7 (dashed line); (b) equivalent Sines stress maps, with maximum value pointed out by an arrow; and (c) experimental failure mechanism, if any

Grahic Jump Location
Fig. 5

Qualitative representation of the mean residual stress due to static contouring with tension/compression at the concave/convex sides and superposition of a time-dependent dynamic stress component due to flexion: these contributions sum in lordotic configuration, overcoming the fatigue limit, while they balance in kyphotic condition

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